Conventional methods of separating water from suspensions, sludges and slurries to improve their solid concentration include mechanical dewatering methods such as centrifugation, vacuum filtration, vibration screening; thermal drying; ponding; ultrasonic dewatering; and use of an electrical field (electrophoretic or electromotic dewatering. Some materials mixed with water, however, are not amenable to the usual dewatering methods due to blockage of the filter by fine particles so that the rate of filtration slows down significantly. In addition dewatering of colloidal and small particle suspensions is difficult with conventional solid-liquid separation techniques such as vacuum filtration or centrifugation. Such suspensions and slurries can also be thermally dried; but, the energy consumption is very high and it creates problems with dust. In addition natural settling by ponding requires large land areas for lagoons and the like.
Because of the high cost of energy there has been a renewed effort to reappraise all of the above methods and reduce the cost associated therewith. The use of an electrical field or acoustical field to separate water from suspended particles has been studied as further discussed below.
When colloidal particles such as finely divided clay are suspended in water they become charged and when subjected to an electrical field they will migrate towards one or another of the electrodes. The charge will depend on the type of suspension or slurry, e.g. certain types of clay become positively charged while many coals become negatively charged, thus the proper electrode polarity will need to be chosen to cause migration of particles away from the water permeable electrode and toward the impermeable electrode.
The application of an electrical field can agglomerate particles by neutralizing charges, dehydrate solids by electroosmosis, or cause the particles to migrate as noted above. The electrically charged collecting plates will sequester all migrating particles such as negatively or positively charged particles but do not collect the isoelectric particles. For example, in proteins, the charges originate from the ionization of (COO.sup.-) and NH.sub.3 +) ions. The net charge on protein will depend on the number of these groups; the disassociation constant, pH, temperature, etc. However, it is an empirical observation that most colloidal protein suspensions are usually negatively charged under normal conditions when in water.
Another phenomenon present is that of electroosmosis. Electroosmosis is the transport of the liquid medium alongside a surface that is electrically charged but stationary. The movement of the liquid medium is in the direction of the electrode with the same sign of charge as the immovable surface. Thus, as the slurry particles become more densely packed and immovable, the water between the particles will be subject to electroosmotic forces. If the particles are negatively charged, the flow of water will be toward the negative electrode. Since this electrode will also be permeable to water, the dewatering process will be further improved.
Illustrative of the use of an electrical field in the dewatering of coal washery slimes is the article by Neville C. Lockhart, Sedimentation and Electro-osmotic Dewatering of Coal Washery Slimes, Fuel, Vol. 60, October, pp. 919-923.
The use of ultrasonic energy separately to dewater coal is known, as illustrated in the articles by H. V. Fairbanks et al., Acoustic Drying of Coal, IEEE Trans. on Sonics and Ultrasonics. Vol. SU-14, No. 4, (October 1967), pp. 175-177; Acoustic Drying of Ultrafine Coals, Ultrasonics, Vol. 8, No. 3 (July 1970), pp. 165-167.
Sonic or ultrasonic energy is a form of mechanical vibratory energy. Sonic or ultrasonic energy propagates as waves through all material media including solids, liquids and gases at characteristic velocities. The wave velocity is a function of the elastic and the inertial properties of the medium.
During the propagation of these waves in a medium very high inertial and elastic forces are generated locally due to the high frequency of these waves. The amplitude of particle motion in the medium due to the sonic and ultrasonic waves range from a few micro inches to 5 milliinches (0.005 inch) (0.127 mm) depending on the power level. The peak acceleration developed in the medium due to an ultrasonic wave at 20,000 Hertz and an amplitude of 0.001 inch (0.024 mm) is as high as 40,000 G(1.5.times.10.sup.6 inch/sec.sup.2) (3.810.times.10.sup.7 mm/sec.sup.2). One G(9.807.times.10.sup.3 mm/sec.sup.2) is the acceleration due to gravity. The forces generated due to these levels of acceleration are very high.
For any medium these high inertia forces generated due to the sonic or ultrasonic waves can cause material failure, disruption and separation. The sonic or ultrasonic impedance of different materials, especially in solid and liquid phases are different by factors of 3 to 8. If the medium is a mixture of different phases of two or more types of materials such as water and coal, etc., the inertia and elastic forces between them are likely to be even higher. These high inertial and elastic forces are likely to break the surface tension and promote separation of liquid from solids.
In liquids a high level of sonic and ultrasonic energy is also known to cause cavitation, a phenomenon of micro bubble formation due to degassing and change of phase to vapors. In the presence of solid particulate matter, the level of cavitation is higher. The micro bubbles are formed on the surface of the solids and assist in the separation of the solid and liquid due to the formation of gas liquid surfaces with much lower surface energy compared to solid liquid surface. Cavitation also generates high local shock waves and in some cases charged free radicals. Shock waves and free radicals are likely to accelerate liquid solid separation.
High oscillatory forces are developed in a medium due to the application of ultrasonic energy. These high oscillatory forces between the solid media and water in a mixture and ultrasonic cavitation are believed to be the major mechanisms of sonic and ultrasonic dewatering. Degassing, decrease of viscosity and decrease of surface tension due to ultrasonic vibration are other possible mechanisms.
Ultrasonic energy is also partially absorbed by the medium and is converted to heat. Internal heat generation and the consequent temperature rise will further decrease the viscosity and the surface tension of the fluid and facilitate its removal. Local temperature rise is also likely to increase the cavitation activity and accelerate the rate of fluid removal. Therefore internal heating due to the partial absorption of the ultrasonic energy has added benefits to accelerate fluid removal as in aqueous systems.
In U.S. Pat. Nos. 3,864,249 and 4,028,232 to Wallis, there are found teachings of the use of acoustical pressure waves and coupling them to a separation screen to facilitate separation of a liquid from material to be dried.
The use of an electrical field or acoustical energy separately requires a substantial amount of energy.
It is an object of this invention to remedy the above drawbacks by reducing the energy requirements for dewatering, by increasing the rate of dewatering over present methods and by lowering the final moisture content of the product. The inventor has discovered that a concurrent use of an acoustical field (sonic or ultrasonic), and an electrical field (electrophoresis/electroosmosis) gives unexpectedly improved results over an acoustical field acting alone or an electrical field acting alone in that:
1. this combination of concurrent use requires less energy than using either of the two alone;
2. this combination of concurrent use dewaters at a faster rate than using either of the two alone; and
3. this combination of concurrent use gives a lower final moisture content to the product than using either of these two alone or using both in sequence.